BOULDER —When Ryan Case was 17, his dad died after an illness that started out as flu and progressed to something worse.

Go to the doctor, Case had told his dad. But his father didn't until it was too late because, after he was laid off from his job as a research chemist, he lost his health insurance and couldn't afford the visit.

Then, in his 20s, Case watched as his mother withered away from multiple sclerosis. The costly drugs she needed to live left her unable to work the job she needed to pay her medical bills. So, Case quit school to work full time to support his mother until her death in 2010.

On Sunday, Case stood before a crowd of 13,000 at the University of Colorado — where he is now a 28-year-old senior — to explain why he is postponing school again. Just one year away from a degree in political science, Case is taking the semester off to work for President Barack Obama's re-election campaign.

Case said he believes Obama's health care plans would provide more affordable coverage to people in situations like his parents'.

"That's not how things should be," Case told the crowd of his parents' health care experiences. "And the president knows that's not how things should be."

"This election," he shouted, "is too important to sit on the sidelines."

The selection of Case to introduce the president at his second rally in Colorado in a week signaled a new push by the Obama campaign to pointedly define the choices in the November election. This week, in the run-up to the Democratic National Convention, the campaign intends to use a series of "American heroes" at campaign events — people like Case who have overcome adversity and can speak to the personal resonance of Obama's proposals — to sharpen the distinction.

On stage Sunday, Obama, at times optimistic and at times combative, showed he is fight-ready.

From health care to energy policy to the economy, Obama time and again attacked the proposals of his Republican opponent, former Massachusetts Gov. Mitt Romney, in the course of offering his vision for the country.

"We believe in an America that says our economic strength has never come from the top down; it comes from the middle out," Obama said to the roaring approval of the crowd.

In an e-mailed response, Romney campaign spokeswoman Amanda Henneberg said one argument Obama's campaign can't make is that Americans are better off now than they were four years ago. She said Obama's speech provided no solutions, while taking a jab of her own — at the bright, blue "Forward" campaign signs rallygoers waived Sunday.

"Instead of taking us 'forward,' President Obama is taking us on a path of declining incomes, high unemployment and trillion-dollar deficits," Henneberg wrote. "The Romney-Ryan plan for a stronger middle class will spur economic growth, bring back jobs and turn our economy around."

During his speech, Obama criticized that plan — the "special sauce," he teasingly called it — for lacking details.

Obama didn't need to work hard Sunday to win over the crowd — a combination largely of students and committed supporters.

"I just want to go vote now," CU student Ali Naaseh, who, at 19 years old, will be voting in his first presidential election, said afterward. "I don't want to have to wait two months."